Jens Stoltenberg said Russia still had a huge force ready to attack Ukraine.
Russia says it is pulling back some of its troops and that military drills in Moscow-annexed Crimea have ended.
But Stoltenberg warned the threat from Russia had become a new normal in European security, adding that Nato was considering setting up new battle groups in central and South East Europe.
He said this was part of ongoing measures to bolster European defence on which two hundred and seventy billion dollars had been spent since 2014 – though he attempted to reassure Russia that Nato was not a threat.
